Foamable compositions comprising high molecular weight engineering thermoplastic resins, such as polycarbonates, polyphenylene ethers, polyesters, and the like, are known, and these can include solid foaming agents and thickeners, such as fumed silica. See, for example, R. B. Allen et al., U.S. Pat. No. 4,544,677.
Japanese Patent Application 47/51821 (1972), reported in Chemical Abstracts 80(6):28011e, discloses expandable polycarbonate compositions including uniformly distributed solid blowing agent, e.g., barium azidocarboxylate. Wetting agents, such as liquid paraffin, are employed to prevent the separation of the solid blowing agent from the resin particles and to stabilize the mixture. Liquid dispersions of solid blowing agents are not disclosed.
Japanese Patent Application 51/138756 (1976), discloses the solid blowing agent, 5-phenyltetrazole, a nylon, and a glass fiber-reinforced polycarbonate and teaches the use of a wetting agent, such as polyethylene glycol or mineral oil, to improve the compatibility of the foamable blend. There is no suggestion, however, to form a liquid dispersion of the blowing agent in the wetting agent.
Now it has been unexpectedly discovered that if liquid dispersion is prepared by mixing a solid, particulate blowing agent, such as 5-phenyl-3,6-dihydro (1,3,4) oxadizin-2-one, with a liquid carrier, such as mineral oil, and an optional thickener, such as fumed colloidal silica, such a liquid dispersion foaming agent can be blended with engineering thermoplastics, such as polycarbonate resins to provide foamable compositions without the need to premix pelletized concentrates, as is done conventionally at the present time. Moreover, there is no problem with lack of compatibility as has been previously experienced. The liquid dispersion can be pumped directly into the hopper of a foam molding machine and the concentration of the solid blowing agent component can readily be changed by changing the pumping rate of the blowing agent delivery system.